1. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany
2. Department of Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany
3. Department of Physics, Atomic and Molecular Physics Laboratory, University of Ioannina, Ioannina 45500, Greece